In February and March 2011, we joined NOAA scientists and other colleagues from around the nation, as well as from Canada and Germany in Erie, Colorado to take part in a month-long air chemistry study with implications for both climate and air quality.

The Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT) study conducted at NOAA’s Boulder Atmospheric Observatory was an effort to decipher why and how the compound nitryl chloride, which is usually associated with the atmosphere near oceans, also forms during the winter nighttime in land-locked regions like the foothills of the Rocky Mountains. Reactions with a variety of airborne contaminants, including smoke from wood burning, pollution from power plants, and road de-icing chemicals, are among the suspects.

Nitryl chloride breaks apart quickly as the sun rises to release chlorine atoms. Chlorine atoms can react with many other compounds, contributing to smog formation, and can also influence chemical cycles that destroy or produce various greenhouse gases, including ozone and methane. The chlorine essentially acts as an accelerant for the photochemistry that leads to ozone production and for the reactions that consume methane. Nighttime formation of nitryl chloride is a gateway to forming more highly reactive chlorine atoms. This “active chlorine” chemistry changes the atmosphere’s starting point for ozone production the next day.

We collaborated closely with Prof. William Keene of the University of Virginia to study the role that soluble gases and aerosol particle composition play in the appearance of nitryl chloride. We employed several instruments, some of which were custom built, to study soluble gases like hydrochloric acid and ammonia, and aerosol particle composition.

Analysis of the data allowed us to estimate the acidity of the aerosol. Our calculations indicate that aerosol particles of all sizes were moderately acidic throughout the duration of the campaign, mostly in the pH range of 2 to 3. (pH is a measure of acidity or alkalinity with 0 being very acidic, 7 neutral and 14 extremely alkaline. The scale is logarithmic; a substance with a pH of 2 is ten times more acidic than one whose pH is 3, etc.) Examples of common substances with pHs in the 2 to 3 range are lemon juice and vinegar.

Aerosol particle pH is important because the nitryl chloride production reactions as we now understand them require that the particles be acidic. These reactions are at the heart of the nighttime chemistry and its impacts on air quality, ozone production, and methane oxidation, which scientists are only beginning to explore. Our data will help to improve mechanistic understanding of the generation and recycling of active chlorine in nocturnal continental air.

The project supported a graduate student who completed a Master of Science degree in 2013 and is now teaching science in a New York City high school. The student’s thesis became the basis for a peer-reviewed paper published in the Journal of Geophysical Research. Our results have also been used in other papers published in the same journal. An undergraduate chemistry student spent one semester assisting with analysis of NACHTT aerosol particle samples by ion chromatography, a widely used method for determining the concentrations of substances dissolved in water solutions. We also published an outreach article about the project in the European magazine “International Innovation”.

Last Modified: 07/31/2014
Modified by: Carolyn Jordan